In recent years, mobile communication systems have become widely popular, and the users have started to demand “availability of communication anytime anywhere”. Accordingly, the mobile IP communication is attracting attention as a technology for satisfying the demand. The mobile IP is an Internet protocol that has been developed for mobile station, which enables the use of the same IP address even when a terminal moves outside a home subnet. The mobile IP communication is described in detail in RFC3344.
FIG. 1 is a diagram illustrating the configuration of a general mobile IP communication system. In FIG. 1, a home agent (HA) 1 manages the address, location, and the like of each mobile node. A foreign agent (FA) 2 has an edge node router function, and is capable of accommodating a mobile node. The home agent 1 and foreign agent 2 are connected via an IP network 3.
Each foreign agent 2 (2a, 2b) broadcasts network information regularly, using a base station. The network information contains information for identifying a foreign agent being the source of the network information. A mobile node 4 performs location registration, when a source of newly-received network information is different from a source of previously-received network information. The location registration is performed as well, when the power of the mobile node 4 is turned on. The procedure for the location registration is described below. It is assumed here that the mobile node 4 performs the location registration with the foreign agent 2a.     (a) The mobile node 4 transmits a location registration request to the foreign agent 2a.     (b) The foreign agent 2a that received the location registration request transfers the location registration request to the home agent 1.    (c) The home agent 1 registers the location (that is, the information for identifying the foreign agent 2a) of the mobile node 4, and returns a location registration response. Accordingly, a communication path (tunnel path) is set up between the home agent 1 and the foreign agent 2a.     (d) A communication path is set up between the foreign agent 2a and the mobile node 4.
After the communication path described above is set up, it is assumed that a correspondent node (CN) 5 transmits a packet to the mobile node 4. At this time, since the IP address of the mobile node 4 is written into a destination address of the packet, the packet is sent to the home agent 1. Then, the home agent 1 transfers the packet to the foreign agent 2a, and the foreign agent 2a further transfers the packet to the mobile node 4. Thus, in the mobile IP communication system, the communication can be carried on using the same IP address, even when the mobile node 4 moves outside the home subnet.
Meanwhile, the mobile IP communication is based on the IP network technology. However, in the IP network, it is difficult to switch communication paths when a congestion or failure occurs. MPLS-TE has known as a technique for avoiding congestion. However, in order to use the method, the MPLS-TE needs to be implemented for each node in a network, and further, a path for avoiding congestion needs to be set up in advance, so the method cannot be adopted easily. For this reason, when congestion or a failure occurs in a mobile IP communication system, it may disable the communication, even though there is a bypass route.
FIG. 2 and FIG. 3 are diagrams describing a problem of the conventional mobile IP communication system. In the mobile IP communication system, generally, in the same manner as in the IP network, only one communication path is set up from a node to its destination node. In other words, only one optimal path is set up between nodes. For example, in the system illustrated in FIG. 2, a foreign agent 2a only has information indicating that the next hop is a router R1, as information directing the path to the home agent 1. In addition, the conventional mobile IP system does not have the function of selecting another path when congestion or failure occurs.
For this reason, when congestion occurs in the optimal path between a foreign agent and a home agent, the location registration may not be carried out, and the communication may not be continued. FIG. 2 illustrates a situation in which congestion occurs in the optimal path from the foreign agent 2a to the home agent 1 (the path routed through a router R1). In addition, in the same manner, when a failure occurs in the optimal path between the foreign agent and the home agent, the communication may not be continued. FIG. 3 illustrates a situation in which a loop is generated at the routers R1, R3 located in the optimal path.
Meanwhile, Patent Document 1 describes a method for avoiding the situation in which the location registration request cannot be performed due to a failure, in a mobile IP communication system. The system adopts a redundant configuration, and aims to shorten the time in which the service is suspended, by continuing the communication using a backup system when a failure occurs in a current system.    Patent Document 1: Japanese Laid-open Patent Publication No. 2004-120411